In a world of ever-increasing energy demands and the need for renewable energy resources, photovoltaics are becoming an increasingly appealing option for energy production [1]. Organic photovoltaic (OPV) cells [2-8] offer a potential alternative to conventional Si solar cells, as exemplified by: i) dye-sensitized cells [9], ii) polymer cells [10], and iii) small-molecule cells [11]. Of these, polymer cells offer the combined attraction of low-cost, light-weight, mechanical flexibility, and amenability to manufacture by high throughput, low-cost, large-area reel-to-reel coating processes. It is expected that such solar cells could be commercially viable if power conversion efficiencies (PCEs) on the order of about 10% were achieved [12]. To date, the highest PCE polymer solar cells have been fabricated with an active layer composed of a blend of regioregular poly(3-hexylthiophene) (P3HT) [13] and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) [14]. The P3HT+PCBM blend forms a phase-separated “bulk-heterojunction” (BHJ) nanostructure which provides a large interfacial area for exciton dissociation. When photo-excited, the P3HT material acts as an electron donor and transporter of holes to the cell anode, while the PCBM material acts as an electron acceptor and transporter of electrons to the cell cathode [10, 15-19]. One material limitation of this BHJ design is the less than optimum match of the narrow P3HT:PCBM optical absorption to the solar spectrum [12]. It is also likely that the multiple, poorly understood interfaces represent a more significant and generic performance constraint to this type of solar cells.
Nanoscale engineering of an anode-organic interface has been successfully implemented in organic light-emitting diodes (OLEDs) for enhancing electrode-organic interfacial physical and electrical contact, resulting in reduced turn-on voltage, blocking of misdirected carriers, enhanced thermal durability, and increased current/power efficiency [20-24]. In BHJ OPV cells, interfacial effects probably limit realization of the maximum theoretical open-circuit voltage (Voc). It is generally thought that the magnitude of Voc parallels the energetic difference between the highest occupied molecular orbital (HOMO) of the BHJ donor material and the lowest unoccupied molecular orbital (LUMO) of the acceptor material [25-28]. This difference, less the exciton binding energy, defines the theoretical maximum Voc, however in actual devices, the output is typically 300 to 500 mV less than this maximum Voc. The hypothesized source of this loss is the field-driven nature of the devices, the presence of dark current, and Schottky barriers formed at the interfaces [28]. One way to enhance OPV performance would then be to suppress these losses to the greatest extent possible. An effective electron-blocking layer (EBL)/hole-transporting layer (HTL) may achieve this goal by preventing current leakage and consequent counter-diode formation [29].
FIG. 1a shows schematically a conventional P3HT:PCBM BHJ solar cell 10 including an active layer 14 of P3HT and PCBM formed an anodal layer 12 that is formed on a glass substrate 11, a lithium fluoride (LiF) layer 15 formed on the active layer 14, and an aluminum (Al) cathodal layer 16 formed on the LiF layer 15. The anodal layer 12 is patterned to have two electrically isolated strips 12a and 12b as an anode, while cathodal layer 16 is patterned to have two electrically isolated strips 16a and 16b as a cathode. Both the anode and the cathode are spatially (electrically) separated. Note that inherent to the conventional BHJ cell architecture 10, both the donor and acceptor materials of the active layer 14 are in direct contact with the anode 12, and it is possible for the acceptor material (PCBM) to transfer electrons to the hole-collecting anode, thereby compromising cell efficiency. Typically, for the P3HT:PCBM solar cell, the PCE is about 2.7-2.9%, where PCE is defined in Equation (1):
                    PCE        =                                            P              out                                      P                              i                ⁢                                                                  ⁢                n                                              =                                                    V                oc                            ⁢                              J                sc                            ⁢              FF                                      P                              i                ⁢                                                                  ⁢                n                                                                        (        1        )            where Pout is the power output of the solar cell, Pin is the power of incident light source (mW/cm2), and Jsc is the short-circuit current density (mA/cm2) of the solar cell.
To prevent electron leakage from the BHJ acceptor to the anode, to aid in photogenerated hole extraction, and to planarize the ITO surface, a thin semiconducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electron-blocking layer (EBL) is typically spin-cast as an aqueous dispersion onto the ITO prior to active layer deposition. This device design has achieved confirmed power efficiencies up to 4% [10]. Despite these positive characteristics, note that aqueous PEDOT:PSS dispersions are at pH about 1 and corrosive to the ITO anode [30, 31]. Furthermore, many researchers find that PEDOT:PSS depositions yield inconsistent film morphologies and electrical properties in accord with the demonstrated electrical inhomogeneity of the films [32, 33]. Finally, polymer light-emitting diode results show that PEDOT:PSS is an inefficient electron-blocking layer, reducing device current efficiency due to electron leakage to the anode [21, 22, 24, 30]. This combination of limitations motivates replacement of PEDOT:PSS by a more suitable material for optimum OPV performance.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.